This invention relates generally to cryogenic technology, and more particularly it relates to cryostats intended for storing biological materials at very low temperatures.
The invention can be used in animal breeding for lengthy storing biological materials, mainly semen of pedigree cattle at cryogenic temperatures, for example, at the temperature of liquid nitrogen; it can also be used in medicine for storing biological preparation such as live tissues, blood, etc, at cryogenic temperatures.
There are known at the present time cryostats comprising a metallic vessel for cryogenic products, inside which various devices for storing biogenic materials can be placed. The vessel is held in a vacuum-tight membrane by its neck and fixed in the required position by stretching in the lower part. The vessel is so placed in the vacuum-tight membrane such that a cavity is formed between them in which heat insulating material is placed. A vacuum valve is installed on the surface of the vacuum-tight membrane to ensure a vacuum in the air-tight insulating cavity.
The vacuum valve is a pipe-branch, one end of which, ending in a filter in the form of a perforated disc, is fixed on the inner surface of the vacuum-tight membrane, while its other end is intended to communicate with the vacuum system during evacuation of the air-tight cavity of the cryostat. As soon as the cavity is fully evacuated, and the vacuum system is disconnected, the pipe-branch is clamped and soldered to ensure tightness of the evacuated cavity of the cryostat.
The vacuum-valve filter ensures a reliable and effective vacuum in the cryostat cavity filled with the powder-type insulation. The insulation material may be on the basis of aerogel of silicic acid, silica gel, silicon, bronze powder, or mixtures of aerogel, silicon and other materials with metallic powders of copper, brass, bronze. The heat conduction coefficient of such vacuum-powder insulation is 0.5.times.10.sup.-.sup.3 - 4.times.10.sup.-.sup.3 kcal/m .times. hour .times. degree; the density of such powders is 100 - 1900 kg/cu.m., the average particle size is from 0.005 to 0.25 mm.
Heat insulation can also be made out of fibre material, for example, on the basis of glass fibre of mineral fibre obtained from melts. The fibres are held together by the natural friction forces, with bakelite varnish, toluene solution of silicon-organic resin, or other binders. Vacuum-fibre insulation has the effective coefficient of heat conduction of about 0.5.times.10.sup.-.sup.3 - 4.5.times. 10.sup.-.sup.3 kcal/m .times. hour .times. degree (at P = 1.times.10.sup.-.sup.3 mm Hg), the density of 150-240 kg/cu.m., the fibre diameter 5-20 micron.
The filter is intended to prevent clogging of the valve, during evacuation of the cavity, with powders and fibres of the insulation used.
However, during evacuation of the insulating cavity, filled with powder insulation, its particles are set in constant motion and can therefore come in contact with the filter to decrease the section of the valve.
Fibres of the vacuum-insulation can also diminish the section of the valve, thus increasing the time of the evacuation process.
At the present time vacuum-multilayer insulation is widely used and is made out of several radiation screens, 0.005-0.02 mm thick, for example, metallic foil or metallized polyethylene terephthalate or polyamide films having high reflecting power (blackness 0.03-0.06), and linings of material having low heat conduction (heat conduction coefficient of 0.01 - 0.04 kcal/m .times. hour .times. degree). However, making use of vacuum-multilayer heat insulation with an (effective heat conduction coefficient of 4.times.10.sup.-.sup.5 - 8.times.10.sup.-.sup.5 kcal/m .times. hour .times. degree) in the known cryostats having the above construction of the vacuum valves, does not ensure reliable and effective evacuation, since vacuum-multilayer heat insulation, during operation of the vacuum system, also closes part of the filter area by clogging its perforations, owing to which the time of evacuation increases. Thus, the production capacity of the vacuum system does not correspond to evacuation of the heat-insulating cavity of the cryostat, due to which the consumption of electric energy by the vacuum system increases. Constructions are possible where the vacuum-multilayer insulation completely blocks the filter and the evacuation of the system becomes impossible.
Moreover, the known design of the vacuum valve does not ensure intactness of the vacuum-multilayer insulation when gas abruptly rushes into the cavity from the environment.
The known vacuum valve provided with a perforated disc does not reduce the flow-rate of the gas jet, and at certain flow-rates layers of heat insulation can be destroyed.